The use of superheterodyne circuits for receivers, in which the received signal is mixed with a local oscillator signal to generate an intermediate frequency signal of above audible frequency, is well known and the invention normally attributed to Edwin H Armstrong who claimed the invention in 1918. These were originally analog technologies.
With the availability and progress of silicon integrated circuit designs, complete radio transmitters and receivers have been integrated onto silicon. The superheterodyne receiver lost favor for high frequency applications due to the narrow bandwidth and high frequency required for the intermediate frequency (IF) filter being difficult to integrate onto silicon. As a result, an architecture originally developed in 1924 by Colebrook was further developed in recent times for integration onto silicon. The homodyne architecture, otherwise known as Zero IF, is shown in the transmitter configuration in FIG. 1. In these systems the local oscillator is at the transmission frequency which allows direct conversion between the baseband and transmission frequencies. The input signal can be analog or digital in this case. The advantage of this system is the ease of integration of the channel filter, which operates at the baseband frequency, at the expense of complexity. The channel filter is a simple low-pass filter, whose bandwidth is half the RF channel bandwidth.
One disadvantage of the zero IF system (among others, including system complexity, DC offsets etc) is that local oscillator phase noise, shown in FIG. 2, degrades the quality of the data. The local oscillator noise 24 is superimposed on the received baseband signals 22 after mixing. The receiver bandwidth is twice the baseband filter 20 as shown. The oscillator noise 24 degrades the baseband signal 22, so is a critical parameter of the design.
The use of an intermediate frequency circumvents the local oscillator noise problem. Typically, the intermediate frequency is set to a frequency that is high enough so that the image frequency may be removed by RF filtering. This leads to the problem that high Q circuits are required to obtain channel selectivity. Another method to circumvent the issues of a zero IF system is to set the intermediate frequency to a frequency where filters may be implemented easily, but the channel does not include the DC component.